Escape $'s in reference default values

[huonw]

This escapes $ in default values to avoid them being treated as template literal substitutions.

Default values end up in a template literal `something`. If the string contains ${...} literally, they'll be treated as a substitution. This PR escapes all $s to avoid this.

This bug/oversight doesn't cause problems with any current values, but will be required in 2.23.0.dev4 due to the new default value for output_path fields from pantsbuild/pants#21175.

Example of the effect (from #234 (comment)):

- default_repr={`'${spec_path_normalized}/${target_name_normalized}${file_suffix}'`}
+ default_repr={`'\${spec_path_normalized}/\${target_name_normalized}\${file_suffix}'`}

[huonw]

Confirming that this "spurious" escaping (i.e. not just immediately before a {, like ${...}), is still fine:

[tdyas]

Thanks for finding and fixing this. Definitely an unknown unknown for me.

[huonw]

Likewise! (just to confirm, this was an underlying problem in the doc processing here, nothing wrong with 21175!)